Thanks for all the suggestions regarding tinplate suppliers. I've looked in to the suppliers mentioned some time ago, KS metals only do small pieces in a very thin gauge. Cliffords have closed. "uktinplate" industry only. The one new supplier to me, Edwards, is promising, waiting on a reply to my inquiry.
It seems that tin is applied by electro methods now, the hot dip being obsolete. I like tinplate for its ease of joining, I've used it for the sheet metal on various models, boiler lagging, water tanks, tender bodies and, in the past, enclosures for my occasional forays into electronics. (I hope Maplins survive, useful range of plastic enclosures stocked) I find the thickness of oil drums too thin, I like a it of thickness to work with!
And thanks Graham W5 for reminding of the thickness grades. How and what was that related to? The remaining small piece I have has XXXXX lightly stamped in one corner.
Paul R, thanks for your offer, I'm 100+ miles away so not practical, unfortunately.
John